1. Field of the Invention
The present invention relates in general to plasma heated arcjet wind tunnels used in the testing of aerospace components and vehicles and, more particularly, to a water-cooled plasma arcjet tunnel segment and method of making same. The arcjet tunnel segments are used to form a wind tunnel used for constricting plasma flow, and effecting an elevation of the temperature in the wind tunnel to simulate conditions encountered in high speed flight.
2. Description of the Related Art
A conventional plasma arcjet heated wind tunnel is typified by the 60-MW shuttle interaction heating facility at Ames Research Center, Moffett Field, Calif., which is a continuous flow electric heated hypersonic wind tunnel system. This facility, which is capable of simulating aerodynamic heating over a broad range of conditions, is shown schematically in FIG. 1. The major components of this facility, shown generally at 1 in FIG. 1, include an arcjet air heater 3 for producing high temperature air, a 60-MW power supply 5, an interchangeable hypersonic Mach number nozzle 7, a walk-in test chamber 11, and an ansillary subsystem consisting of a steam-ejector vacuum system 9, an air control system 13, and a cooling water system 15, 11 (FIG. 1).
A primary component of such a facility is the high power, constricted arc heater for producing high temperature air for the nozzles. In such heaters, a long high voltage discharge occurs between two electrode assemblies. The arc current is high so that it becomes constricted by the confining constrictor tube. This constriction causes the arc-column temperature to rise to very high levels, for example, to 10,000° K. The constrictor tube consists of a plurality of discs, which may be fabricated in modular form and comprise, for example, thirty discs—each only 1 cm thick and 8 cm in diameter. In the 60-MW plasma arcjet tunnel the constructor tube is 3.9-m large.
A clear plastic material may be used as a potting agent on each module holding the discs in position. The assembly is easily sealed for high pressure operation, and possesses good dielectric properties for avoiding arc-over due to high voltage gradients.
A conventional constrictor-disc is shown generally at 17 in FIG. 2. The disc 17 is water cooled by a source of water entering at 19 and exiting at 21. The cooling water flows from peripheral entrance 23 of the outer disc 25 to passageway 27, and then into and around a cusp-shaped water passageway 29 formed in inner ring 31. The purpose of the cusp is to induce a centripetal pressure gradiant within the water cooling passage 29. This gradiant, thought to be essential, causes nucleate boiling bubbles to rise from the surface.
The cross-sectional view, FIG. 3, taken along line 3-3 in FIG. 2, illustrates the flow of air into the constrictor disc through air passage 33. A disc filler 35 is shown in FIG. 3 adjacent to outer disc 25, as well as o-ring seals 37, and backup ring (insulated) 39. The air flow out is shown at 36. Operating experience at high heating conditions evolved the construction techniques using the undulating cusp-shaped water passage 29, and a one-piece spool and filler design shown in the sectional disc assembly shown in FIGS. 2 and 3.
Conventional water-cooled arcjet tunnel segments are disc shaped pieces of copper that provide physical spacing between an anode and cathode in a plasma heated wind tunnel. The physical spacing between the anode (at the inlet of the tunnel) and cathode (at the exit of the tunnel) allows a significant electrical potential to occur, which in turn, is discharged in the tunnel in the form of a plasma arc. This arc heats the incoming high-pressure working fluid, which is usually air, but can be helium, or a mixture of gases. The tunnel segments are water-cooled to survive in the demanding thermal environment produced through the proximity of the segments to the plasma arcjet process. A high pressure flow of the working fluid is introduced between the segments in a swirling fashion to aid in centering the arcjet and cooling the segments, while also providing a significant portion of the total working fluid that is ejected from the hypersonic nozzle.
The primary disadvantage of the current design, as shown in FIGS. 2 and 3, is in its fabrication requirements which requires the use of a silver solder, and vacuum oven brazing. This process is often referred to as “black magic” in the aerospace industry due to the high degree of personal experience required by the fabricator to produce repeatable and reliable products from production run to production run. Depending on the complexity of the geometry of the parts that are to be joined (e.g. area to be brazed, braze interface, braze gap, etc) and the material to be brazed, the degree of difficulty in the vacuum brazing goes up significantly with a commensurate increase in the number of scrap or failed parts. As the number of failed parts increases, the cost per satisfactory part increases as well.
In the case of the current segment design, three (3) parts are brazed together requiring a very deep turning operation and a high level of alignment to ensure that there is a consistent and properly sized braze joints to allow the silver solder to flow at temperature. Positioning the silver solder foil between the individual parts and ensuring that the solder flows evenly in the gap is a substantial hurdle to low part count rejection rates. Repeatability of the braze joint interface, alignment and foil placement are highly subject to human error, and, are therefore highly dependent on the fabricator. In addition, the vacuum oven process itself requires specific tailoring and manipulation on a part by part basis to ensure the proper process variables (e.g., vacuum level, oven temperature, time, etc.).
It is therefore an object of the present invention to provide a plasma arcjet tunnel segment and process of producing same that eliminates this level of individual fabricator experience, and improves the repeatability of the fabrication process therefor.
It is another object of the present invention to provide improved plasma arcjet tunnel segments and process of producing same which eliminates welding slag and brazing materials flow into the water channel, and which segments can withstand the high thermal and pressure extremes encountered in operation.
It is yet another object of the present invention to provide a plasma heated arcjet wind tunnel with tunnel segments in the arc constrictor which overcome operating problems of the conventional tunnel segments.